Quantum Well Intermixing (QWI) is a process which has been reported as providing a possible route to monolithic optoelectronic integration. QWI may be performed in III–V semiconductor materials, eg Aluminium Gallium Arsenide (AlGaAs) and Indium Gallium Arsenide Phosphide (InGaAsP), which may be grown on binary substrates, eg Gallium Arsenide (GaAs) or Indium Phosphide (InP). QWI alters the band-gap of an as-grown structure through interdiffusion of elements of a Quantum Well (QW) and associated barriers to produce an alloy of the constituent components. The alloy has a band-gap which is larger than that of the as-grown QW. Any optical radiation (light) generated within the QW where no QWI has taken place can therefore pass through a QWI or “intermixed” region of alloy which is effectively transparent to the said optical radiation.
Various QWI techniques have been reported in the literature. For example, QWI can be performed by high temperature diffusion of elements such as Zinc into a semiconductor material including a QW.
QWI can also be performed by implantation of elements such as silicon into a QW semiconductor material. In such a technique the implantation element introduces point defects in the structure of the semiconductor material which are moved through the semiconductor material inducing intermixing in the QW structure by a high temperature annealing step.
Such QWI techniques have been reported in “Applications of Neutral Impurity Disordering in Fabricating Low-Loss Optical Waveguides and Integrated Waveguide Devices”, Marsh et al, Optical and Quantum Electronics 23, 1991, s941–s957, the content of which is incorporated herein by reference.
A problem exists with such techniques in that although the QWI will alter (increase) the band-gap of the semiconductor material post-growth, residual diffusion or implantation dopants can introduce large losses due to the free carrier absorption coefficient of these dopant elements.
A further reported QWI technique providing intermixing, is Impurity Free Vacancy Diffusion (IFVD). When performing IFVD the top cap layer of the III–V semiconductor structure is typically GaAs or Indium Gallium Arsenide (InGaAs). Upon the top layer is deposited a silica (SiO2) film. Subsequent rapid thermal annealing of the semiconductor material causes bonds to break within the semiconductor alloy and Gallium ions or atoms—which are susceptible to silica (SiO2)—to dissolve into the silica so as to leave vacancies in the cap layer. The vacancies then diffuse through the semiconductor structure inducing layer intermixing, eg in the QW structure.
IFVD has been reported in “Quantitative Model for the Kinetics of Compositional Intermixing in GaAs—AlGaAs Quantum—Confined Heterostructures”, by Helmy et al, IEEE Journal of Selected Topics in Quantum Electronics, Vol 4, No 4, July/August 1998, pp 653–660, the content of which is incorporated herein by reference.
Reported QWI, and particularly IFVD methods, suffer from a number of disadvantages, eg the temperature at which Gallium out diffuses from the semiconductor material to the silica (SiO2) film.
It is an object of at least one aspect of the present invention to obviate or at least mitigate at least one of the aforementioned disadvantages/problems in the prior art.
It is also an object of at least one aspect of the present invention to provide an improved method of manufacturing an optical device using an improved QWI process.